IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) and Session Description Protocol (SDP) to set up and control calls or sessions between user terminals (or user terminals and application servers). Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control user access to services and to charge users accordingly.
FIG. 1 illustrates schematically how the IMS fits into the mobile General Packet Radio Service (GPRS) network architecture. As shown in FIG. 1 control of communications occurs at three layers (or planes). The lowest layer is the Connectivity Layer 1, also referred to as the bearer plane and through which signals are directed to/from user equipment (UE) accessing the network. The entities within the connectivity layer 1 that connect an IMS subscriber to IMS services form a network that is referred to as the IP-Connectivity Access Network (IP-CAN). The GPRS network includes various GPRS Support Nodes (GSNs). The middle layer is the Control Layer 4, and at the top is the Application Layer 6.
The IMS 3 includes a core network 3a, which operates over the middle, Control Layer 4 and the Connectivity Layer 1, and a Service Network 3b. The IMS core network 3a includes nodes that send/receive signals to/from the GPRS network and network nodes that include Call/Session Control Functions (CSCFs) 5. The CSCFs 5 include Serving CSCFs (S-CSCF) and Proxy CSCFs (P-CSCF), which operate as SIP proxies within the IMS in the middle, Control Layer 4. Other IMS core network entities shown include a Media Resource Function Controller (MRFC), a Border Gateway Control Function BGCF and a Media Gateway Control Function, (MGCF). The top, Application Layer 6 includes the IMS service network 3b with Application Servers (ASs) 7 for implementing IMS service functionality.
As shown in FIG. 1, a User Equipment (UE) can access the IMS by attaching to an access network and then over the Connectivity Layer 1, which is part of a Packet Switched (PS) domain. For example, the UE may attach via an Evolved Packet Core (EPC)/Long Term Evolution (LTE) access. In that case an IMS session can be set up by the UE using SIP signalling. However, a UE may also access IMS services via a Circuit Switched (CS) domain 8. Although the CS domain will not handle SIP, procedures are well established for dealing with the provision of media and services between the IMS and a UE using a CS access.
3GPP has established standardised procedures specified in Technical Specifications (TS). For example TS 23.228 relates to IMS SIP, TS 23.203 relates to the Policy and Charging Control (PCC) architecture, and TS 23.167 relates to Emergency Calls for IMS. In addition the so-called Single Radio Voice Call Continuity (SRVCC) specifications (TS 23.216 and TS 23.237) deal with the handover of calls from a PS to a CS access network, and specify how emergency calls are routed.
The 3GPP Technical Specifications stipulate that when an emergency call is made to an emergency centre such as a Public Safety Access Point (PSAP), special procedures are applied to ensure that the call is correctly routed and not interrupted. However, there may be situations where the PSAP might want to call-back the UE after the call has been terminated (intentionally or unintentionally terminated). As specified in the standards, an emergency call that is established over the IMS is anchored in the IMS entities that serve the UE through the access network at which the UE was attached when the emergency call was established (i.e. in the visited network, not in the user's Home Network as would be the case for a normal call).
The current procedures for call-back to a particular device require the PSAP to call back to the right device by indicating an equipment identifier or instance ID or Globally Routable User Agent Uniform Resource Identifier (GRUU) provided by the UE during emergency call establishment (see TS 24.229). However, this requires an IP-PSAP with enhanced functionality, something unlikely to be deployed in many countries.
WO03049467 A1 discloses a method and system for call-back in case of an emergency session. The document suggests an emergency call setup similar to the setup of a normal call, thus involving an S-CSCF located in the home network of the UE making the emergency call. However, shown system result in an inefficient emergency call setup that is not in accordance with preferred procedures for emergency call setup.